Earthworms are invertebrate animals that play an important role in soil ecology. They have a closed circulatory system, a simple nervous system, and a segmented body with many specialized structures. One of the structures used for excretion in earthworms is called the nephridium.
The nephridium is a tubular structure found in each body segment of an earthworm. It functions as the excretory organ of the earthworm and is responsible for filtering waste products from the blood and removing them from the body. The nephridium is similar in function to the kidneys of mammals.
The process of excretion in earthworms begins with the coelomic fluid, which is a fluid that fills the body cavity of the earthworm. The nephridium filters the coelomic fluid, removing waste products such as nitrogenous compounds and excess water. The waste products are then transported to the outside of the body through small openings called nephridiopores.
In contrast, the other structures listed in the options are not used for excretion in earthworms. The contractile vacuole is used by some aquatic organisms to expel excess water from the body, the flame cell is used by some flatworms for excretion, and the Malpighian tubule is used by some insects to remove waste products from the body.
Therefore, the correct answer is nephridium, which is used for excretion in earthworms.